The invention relates generally to a semiconductor device, and more specifically to a bond pad structure overlying a substrate.
FIG. 1A shows a cross-section of a conventional bond pad structure 1 with a conductive layer 110 and dielectric layer 120 on a substrate 100. The substrate 100 has a plurality of active devices and interconnections (both not shown). The conductive layer 110 is usually copper, electrically connecting to the interconnections of the substrate 100. A dielectric 120 isolates the conductive layer 110 from unwanted electrical connection to other devices (not shown). A passivation layer 150, with an opening 152 exposing the conductive layer 110, is formed on the dielectric layer 120 and conductive layer 110. The conductive layer 110 is an I/O port to electrically connect the substrate 100 to an external device (not shown). When the conductive layer 110 is copper, corrosion will start at the grain boundaries on the surface of the conductive layer 110 and proceed along the grain boundaries deeper into the conductive layer 110 when exposed to the atmosphere. Therefore, a metal layer 140, usually aluminum-copper alloy, overlying the conductive layer 110 is normally necessary to protect the conductive layer 110 from corrosion. A barrier layer 130 between the conductive layer 110 and metal layer 140 prevents unwanted inter-diffusion between the conductive layer 110 and metal layer 140 as needed. A passivation layer 160, with an opening 162 exposing the metal layer 140, is formed on the passivation layer 150 and metal layer 140. Both passivation layers 150 and 160 protect the substrate 100 from damage from moisture, oxygen, particles, and other corrosive factors or contaminants. Both passivation layers 150 and 160 further isolate the metal layer 140 from unwanted electrical connection to other devices (not shown).
When the wafer fabricating process is complete, a wafer probing process is performed to test functions of the substrate 100. In FIG. 1B, the wafer probing process is shown. A probe 180, usually tungsten, is provided to contact the bond pad structure 1. The probe 180 is needle-like and much harder than the metal layer 140 and conductive layer 110, so the probe 180 can penetrate the metal layer 140 into the conductive layer 110. The probe 180 may further slide during probing from vibration of the testing apparatus (not shown) or other factors. Therefore, the metal layer 140 may be completely or partially peeled from the conductive layer 110.
In FIG. 1C, the metal layer 140 is partially peeled from the conductive layer 110 after the wafer probing process, forming an opening 170 exposing parts of the conductive layer 110. Furthermore, the bond pad structure 1 may be tested a number of times for different functions, so multiple openings 170 may be formed. Thus, the exposed conductive layer 110 may corrode from the reaction with oxygen, moisture, and/or other corrosive factors, forming a corrosive layer 172 thereon when the substrate 100 is exposed to the atmosphere.
When the substrate 100 is packaged, the corrosive layer 172 often further negatively affect the yield of the packaging process or reliability of the complete package. In FIG. 1D, a gold wire 190 with a gold ball 192 is bonded to the bond pad structure 1, specifically on the metal layer 140. The gold ball 192 cannot bond to the corrosive layer 172, reducing the effective bonding area between the gold ball 192 and the bond pad structure 1 and weakening the bonding strength therebetween. The gold ball 192 may separate along the corrosive layer 172, creating bond-off-pad defect, during the molding step of the packaging process from the mold flow. If the gold ball 192 does not separate during the molding step, subsequent thermal steps of the packaging process or the complete package may also cause separation on the gold ball 192.